Unidirectional second order gradient microphone

Abstract

A second order gradient microphone with unidirectional sensitivity pattern is obtained by housing each of two commerically available first order gradient microphones centrally within a baffle. The baffles have flat surfaces, are preferably square or circular and have parallel surfaces the two baffles being parallel to each other. The rotational axes of the microphones are arranged to coincide. The output signal from one of the microphones is subtracted from the delayed signal output of the other.

What is claimed is: 
     
       1. A microphone arrangement for use with incident wave energy whose frequency varies from a first to a second frequency, said second frequency being substantially greater than said first frequency and the first and second frequencies respectively corresponding to a first and a second wavelength, said arrangement comprising a plurality of first order gradient microphones, each microphone having first and second sensing surfaces separated by a first acoustical path, and each microphone providing an output signal;   a plurality of baffles, each baffle receiving at least one associated microphone, each baffle blocking said first acoustical path of each associated microphone and creating a second acoustical path, said second acoustical path being greater than said first acoustical path and substantially less than said second wavelength; and   means for combining the output signals of said microphones to provide a second order directional response pattern for said microphone arrangement.   
     
     
       2. The arrangement of claim 1 wherein said combining means adds the output signals from said microphones. 
     
     
       3. The arrangement of claim 1 wherein said combining means subtracts the output signals from said microphones. 
     
     
       4. The arrangement of claim 1 wherein said combining means includes at least one delay element which delays an associated one of said output signals. 
     
     
       5. The arrangement of claim 1 wherein said first and second sensing surfaces are parallel to one another. 
     
     
       6. The arrangement of claim 1 wherein said sensing surfaces of each microphone have a different one of two polarities. 
     
     
       7. The arrangement of claim 6 wherein said microphones are arranged with sensing sufaces of the same polarity facing one another. 
     
     
       8. The arrangement of claim 6 wherein said microphones are arranged with sensing surfaces of different polarities facing one another. 
     
     
       9. The arrangement of claim 1 wherein a pair of microphones is substantially parallel and separated by a distance less than said second wavelength. 
     
     
       10. The arrangement of claim 1 wherein each microphone is centrally disposed within an associated baffle. 
     
     
       11. The arrangement of claim 1 further including at least one additional baffle connected to said microphones, each additional baffle being greater in size than said second wavelength. 
     
     
       12. A method of producing a directional sensitivity pattern for a microphone arrangement for use with incident wave energy whose frequency varies from a first to a second frequency, said second frequency being substantially greater than said first frequency and the first and second frequencies respectively corresponding to first and second wavelengths, said method comprising the steps of perforating a recess through each baffle in a pair,   placing a bidirectional first order microphone having first and second sensing surfaces separated by a first acoustical path into each recess, the placement of each microphone replacing said first acoustical path with a second acoustical path, said second acoustical path being substantially less than said second wavelength, each microphone providing an output signal, and   combining the output signals of each microphone.   
     
     
       13. The method of claim 12 further comprising the step of introducing a delay into the output of at least one of said microphones prior to combining the microphone output signals.